is it possible to dynamically set the level of for loop nesting

Question

I'm working out an algorithm to get permutations like

123
132
213
231
312
321

I'm doing it using nested foreach loops.

for (..) {
    for(..) {
        for(..) {
           echo $i . $j . $k . "<br />";
        }   
    }
}

Problem is those # of nested loops are optimized for 3-spot permutations. How can I could I dynamically set the number of nested for loops to generate 4-letter or 5-letter permutations?

Answer 1

The answer is: use a recursive function.

Answer 2

You could use a recursive function. Take a look at this post.

Answer 3

Recursive method is the way to go. But you can also use eval function like:

$loop = iteration('i',10) . iteration('j',10). iteration('k',10). iteration('l',10);
$loop .= "print \"\$i \$j \$k \$l\\n\";";

// $loop now has: for($i=0;$i<10;$i++)for($j=0;$j<10;$j++)for($k=0;$k<10;$k++)for($l=0;$l<10;$l++) print "$i $j $k $l\n";
eval($loop);

function iteration($var,$limit) {
    return "for(\${$var}=0;\${$var}<$limit;\${$var}++)";    
}

Answer 4

Yes, just do it recursively.

function permuteThis($items, $permutations = array()) {

    if(!is_array($items))
        $items = str_split($items);

    $numItems = sizeof($items);

    if($numItems > 0) {
        $cnt = $numItems - 1;
        for($i = $cnt; $i >= 0; --$i) {
            $newItems   = $items;
            $newPerms   = $permutations;
            list($tmp)  = array_splice($newItems, $i, 1);
            array_unshift($newPerms, $tmp);
            permuteThis($newItems, $newPerms);
        }
    } else {
        echo join('', $permutations) . "\n";
    }
}

$number = 123;
permuteThis($number);

Answer 5

No, no, please do NOT use recursion for generating permutations. Use the algorithm outlined here, see also php implementation